In the oil drilling industry, there are two traditional methods of drilling an oil well. One is to attach a drill bit at the end of a drill string, apply downward pressure, and rotate the drill string from the surface so that the drill bit cuts into a formation. The problem with this method is that as the hole becomes deeper and the drill string becomes longer, the frictional forces due to the rotation of the drill string down hole increase, especially in deviated and horizontal wells.
The second method is to place a motor down hole near the drill bit. This method requires a special type of motor (or pump) called a positive displacement motor, or PDM. The PDM is also referred to in the oil drilling industry as a Moineau pump or mud motor. It has a long spiral rod inside of it, called a rotor, which spins inside of a stator as fluid is continually pumped down the drill string through the motor. The speed at which a mud motor rotates depends upon the internal geometry of the motor, the flow rate of the fluid that is pumped down the drill string to turn the motor, and the resistance of the formation against the drill bit. Although the pumping of the fluid down the drill string is one factor that determines the speed at which the drill bit rotates, the circulation of the drilling fluid serves other purposes as well. For example, it circulates the cuttings out of the hole and cools the drill bit as it cuts into harder formations.
When drilling a hole, an operator frequently encounters the need to change the rotational speed of the drill bit. When drilling through harder, more difficult formations, slower bit speeds are required. When encountering softer formations, an operator may select a faster drill speed to drill quickly through the formation. If an operator cannot change the flow rate of the fluid pumped down the drill string because, for example, the operator needs to maintain some minimum flow rate to circulate the cuttings out of the hole, then the only other option to change drill speeds is to change the internal geometry of the motor.
Prior art motors do not have the ability to change their internal geometries down hole without bypassing a portion of the fluid flow outside the drill string. This has at least two deleterious effects. First, not all of the fluid pumped down a drill string will pass through the drill bit to cool it, and, second, not all of the fluid flow pumped down the drill string will be used to circulate the cuttings out of the hole.
One way to overcome these problems is to remove the drill string from the hole and replace the motor with one having a different internal geometry or to modify the internal geometry of the motor used. The removal of the drill string to replace a motor is time consuming and expensive. Consequently, there is a need in the art for a method and/or apparatus that allows an operator to change the internal geometry of mud motors down hole without passing a portion of the fluid flow outside the drill string.